1. Field of the Invention
This invention relates generally to a fluid bed apparatus and, in particular, to an improved gas distribution plate of the fluid bed apparatus which plate provides enhanced gas distribution flow to the reaction chamber of the apparatus and wherein the gas distribution plate prevents backsifting of bed particles from the fluid bed reaction chamber to the windbox chamber situated below the gas distribution plate.
2. Description of Related Art
Fluid bed apparatus are well known in the art and have been used for a number of industrial processes ranging from chemical reactions to the heating and/or cooling of materials. Basically, the fluid bed apparatus comprises a chamber through which a gas is passed upwardly and the gas fluidizes solid particles in the chamber and forms a fluid bed which is typically characterized as being similar to a boiling liquid.
A typical fluid bed apparatus comprises a vertical elongated chamber having a gas distribution plate near the bottom of the chamber. Air or other gaseous material (hereinafter referred to as gas for convenience) is introduced into a windbox or gas distribution chamber below the gas distribution plate and flows upward through openings in the plate and mixes with and fluidizes particulate solid material which is contained in the reaction chamber above the gas distribution plate. When the flowing gas and solid particles are mixed a fluid bed is generated which may be characterized as a very turbulent, random moving mixture. Depending on the operation performed in the fluid bed apparatus, the fluidizing gas, typically air or nitrogen, exits at the top of the fluid bed apparatus through a flue. The space in the chamber above the fluidized bed is typically called the freeboard section where larger solid particles entrained in the gas typically separate by gravity from the rising gas stream and fall back into the fluidized bed. The gas exiting the flue usually contains some fine solid particles. Where it is necessary to remove a product from the apparatus, outlets at various parts of the apparatus are provided to remove the particles which have either been reacted, dried, cooled or otherwise treated.
It is important that the gas fed into the fluid bed apparatus and which is mixed with the solid particulate matter in the reaction chamber be evenly distributed to provide a uniform and efficient fluidized bed mixture. Typically, such uniformity is attained by using a gas distribution plate having a number of evenly spaced vertical openings across the width of the plate through which the gas travels and mixes with the solid bed particles. As can be appreciated, one of the problems with a fluid bed apparatus is that the bed particles in the chamber, which are typically fine, may fall back (backsift) through the openings in the gas distribution plate into the windbox chamber. The backsifting may not only clog the holes inhibiting or preventing gas flow therethrough but also decreases the efficiency of the operation since the backsifted particles are not in the reaction chamber and will have to be removed from the gas windbox chamber and recycled back to the fluid bed in the reaction chamber. This problem is especially serious for fluid bed solid materials which are highly flowable, small in size and have a minimum or zero angle of repose.
To overcome the problem of backsifting of bed particles below the gas distribution plate, various designs for the gas distribution plate have been developed. One of the designs utilizes adjustable ports to restrict the backsifting and such a gas/diffusion plate for a fluidized bed apparatus is described in U.S. Pat. No. 4,068,389, which patent is hereby incorporated by reference. This patent shows the use of screw studs which are screwed into openings in the gas distribution plate and wherein the openings are tapped. The screw studs are designed to provide a tortuous path of small cross-sectional area between the side of the opening and the screw stud in which the gas travels from the windbox chamber to the fluid bed chamber of the apparatus. A modified embodiment of the screw stud is the use of a machine screw having a head wherein interposed between the bottom face of the head and the upper surface of the distribution plate is a standard form of internal lock washer through which the gas passes after exiting the screw thread opening. This embodiment is preferred over the screw stud since the lock washer is in a horizontal plane whereby agitated particles are precluded from entering due to gravity. It has also been proposed to use sintered metal plates as the gas distribution plate in a fluid bed apparatus but sintered plates are typically fragile and crack due to thermal expansion stresses. Size and temperature will be limited for this type of design.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a highly effective and efficient gas distribution plate for use in a fluid bed apparatus which minimizes backsifting of solid bed particles below the plate into the gas chamber or windbox of the apparatus.
It is a further object of the present invention to provide a fluid bed apparatus having an improved gas distribution plate which plate minimizes backsifting of solid bed particles from the reaction chamber to the gas chamber or windbox of the apparatus while still providing highly effective and efficient gas distribution to the reaction chamber of the apparatus.
It is yet another object of the present invention to provide a method for operating a fluid bed apparatus wherein there is a minimum of backsifting of bed particles from the reaction chamber to the gas chamber or windbox of the apparatus.
Another object of the present invention is to provide a tuyere for use in the gas distribution plate of a fluid bed apparatus whereby gas flows upward through a plurality of the tuyeres and fluidizes solid particulate matter in the apparatus without any significant backsifting of solid particles through the tuyeres.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.